Porphyrin–triarylamine conjugates: strong electronic communication between triarylamine redox centers via the porphyrin dication

Abstract

A set of porphyrin–triarylamine hybrids have been synthesized in good yield by Sonogashira palladium-catalyzed cross-coupling reactions between the zinc complex of 5,15-diethynyl-10,20-dimesitylporphyrin and the appropriate iodophenyldiarylamines. The crystal structure of porphyrin 1 shows that the dihedral angle between the acetylene-bonded benzene rings and the porphyrin macrocycle is 20.0°. Such a structural characteristic enables effective electronic perturbations within the molecule. The electronic spectra are red-shifted and display a broad Soret band and an intense Q band relative to those of meso-substituted tetraarylporphyrins. These conjugates display four oxidations and one reduction. All the electrochemical reactions involve one-electron transfer. The first and second oxidations are reversible and can be assigned to the porphyrin-centered reactions. The third and fourth ones, separated by about 270 mV, correspond to the triarylamine units. The comproportionation constant (K_c) is calculated to be 3.67 × 10⁴. The electron coupling between the triarylamine moieties, at a separation of >23 Å, is remarkably strong. The electrochemical results and the absorption spectra show that the electronic characteristics of these porphyrins can be significantly modulated by the triarylamine substituents via the conjugated carbon–carbon triple bond. Variations of the substituents on the triarylamines can fine-tune the electronic properties of these molecules.